1. Field of Invention
This invention relates to safety devices for horizontal boring or excavating operations, specifically to an alerting or warning method and device that indicates that an electrical power source has been contacted and its insulation broken.
All equipment used to excavate ground cavities have a potential hazard of damaging buried electrical cables, and as more cables are buried the probability increases. The process of horizontal boring is particularly hazardous because it is usually employed to make a tunnel or conduit for installation of lines under an existing structure, road, or private lawn where power lines are common.
2. Discussion of Prior Art
It has been an erroneous general feeling and teaching that a metal rod buried in earth will be grounded and will cause circuit protection devices to open if the rod contacts a power transmission line. OSHA, UL, and NEC codes and standards have indicated an acceptable ground rod will have an impedance of 25 ohms or less. At 25 ohms, a 115 volt house line will only draw less than 5 amps. Field measurements on horizontal bore strings buried over 25 feet in length show impedances back to utility power returns of up to several hundred ohms. Vertical rods usually have lower impedances.
Earth has a very high impedance to the flow of electricity compared to the low impedance of most metals. Any metal that contacts an electric transmission line even if on top of or buried in earth will attain the voltage level of that line.
If the earth impedance is high, only small currents will flow and the hazard potential will be great. Fuses and circuit protection may not turn the power off. Also large voltage gradients through the earth may present hazards to bystanders and workers standing on the ground some distance from the equipment.
If the earth impedance is low, large currents will flow and fuses, breakers, or switch gear will trip. On-site workers may not feel or see any sign of the contact, or conversely it is possible the jolt will be lethal. If switchgear was tripped, it may automatically reclose after a few seconds.
With either high or low impedance earth, there will be a voltage difference or gradient between the line-contacting metal and nearby earth irrespective of any attempts to `ground` the equipment. There will also be voltage gradients through the earth as a function of the space or distance from the equipment to line contact point. Therefore, sensing the gradient from the equipment to earth or from close earth to farther earth serves the same purpose and provides a reliable indication of contact or breeched insulation.
The current sensing alarm in U.S. Pat. No. 4,755,805 to Chau, 1988 Jul. 5, requires an equipment grounding point sufficient to reduce voltages at the equipment to a negligible level. Since switchgear typical in shopping malls are designed to open lines drawing 80,000 amps, an adequate ground point and wire would have to have much less than 1 milliohm for a negligible voltage at the equipment. A sensing coil used to detect the current would in itself defeat the ability to obtain a low resistance ground.
Frequently, when boring or excavating, a power line will be hit a momentary glancing blow removing insulation but not causing major burns or other indication of contact. Also, a line may be protected with fuses or switchgear that trip quickly. In either case, Chau's horn, of the above referenced patent, would not sound. A subsequent movement, return of the bit, or closing switchgear might cause a hazard. In the device of the present invention, the horn is latched in its activated state with even a momentary voltage and must be manually reset after all hazards are known to be cleared.
Therefore, it is an objective of this invention to provide a method and a device for announcing contact of a boring bit or excavation tool with an in-ground source of electricity such as a buried power line.
An additional objective is to provide a continuing signal if the contact with an electrical source is only momentary.
Another objective of this invention is to provide a signaling device that does not require on-site measurement, calibration or provision of a special equipment ground.
A further objective of this invention is to provide a conveniently small and portable device that can be mounted on excavating or boring equipment or that can be carried separately as an independant device that may be used with portable tools such as digging bars, shovels or any other earth penetrating equipment.
Other objects and advantages of this invention will be apparent after consideration of the drawings and descriptions.